1. Field of the Invention
The present invention relates to an ultrasonic observation apparatus that generates transmit pulses suitable for exciting an ultrasonic transducer.
2. Description of the Related Art
In recent years, an ultrasonic observation apparatus is connected to an ultrasonic endoscope or an ultrasonic probe to perform a substantial diagnosis of the degree of submucosal invasion of a change or an organ.
The distal end of the ultrasonic endoscope or ultrasonic probe internally contains an ultrasonic transducer. The electric driving pulses transmitted from the ultrasonic observation apparatus and applied to the ultrasonic transducer are converted to acoustic ultrasonic pulses by the ultrasonic transducer and are irradiated to an internal tissue.
The reflected waves returned from the internal body are converted to electric signals by the ultrasonic transducer, undergo signal processing and are displayed as an ultrasonic tomographic image.
Conventionally, PZT (two-component system piezoelectric ceramics Pb(Ti,Zr)O3) transducer or a complex piezoelectric element is used as the ultrasonic transducer, and a driving method suitable for the ultrasonic transducer has been adopted.
For example, the fractional bandwidth of a conventional PZT transducer is about 70%, and pulses having a time slot of the center frequency are driven by about three burst waves (continuous waves) in order to use the PZT transducer efficiently.
The complex piezoelectric element has a significantly wide fractional bandwidth of 100% or larger, compared to a conventional PZT transducer.
In order to use such an ultrasonic transducer, the frequency band of transmit pulses largely has increasingly largely depended on an ultrasonic image thereof since the frequency bandwidth of the complex piezoelectric element is wider than the frequency bandwidth of the transmit pulse.
For example, THI (Tissue Harmonic Imaging) has gathered attentions as a method for improving the lateral resolution.
According to the technology, when ultrasonic signals of fundamental waves are transmitted from the ultrasonic transducer, the fundamental waves are distorted on a propagation path within a body, and second- and third-order harmonics occur.
Imaging extracted signals of the harmonics occurring in the internal body is called THI.
A technology for transmitting fundamental waves only to the internal body and a technology for receiving the harmonics occurring in the internal body are important for performing THI. Therefore, the ultrasonic transducer must have a wider fractional bandwidth, and an ultrasonic transmitting circuit that generates ultrasonic fundamental waves must have a configuration that prevents the occurrence of harmonics.
The ultrasonic observation apparatus has a problem unique to medical equipment.
An ultrasonic endoscope or ultrasonic probe must be inserted to the inside of a human body, and, in order to assure the security for a human being from the point of view, the standards relating to electric security on leak current and withstand voltage, for example, must be met.
In order to be satisfied with the standards on the leak current and withstand voltage, a conventional ultrasonic observation apparatus must have a patient circuit that floats a circuit to which an ultrasonic endoscope or ultrasonic probe is electrically connected, such as a transmitting circuit part from a primary circuit (commercial power supply) of the ultrasonic observation apparatus and a secondary circuit (including the apparatus cabinet) that operates the inside of the apparatus.
The ultrasonic observation apparatus must have the patient circuit and, at the same time, keep the amount of noise radiated to the outside of the apparatus (radiated electromagnetic noise) equal to or lower than a provided value.
The eradiated electromagnetic noise is restricted so as to prevent an adverse effect to equipment used in a medical organization, such as a pacemaker.
In order to reduce the radiated electromagnetic noise, the current to be supplied to the patient circuit must be kept as small as possible.
Weakening the electromagnetic noise by a current loop in a size reduced as much as possible within a circuit by reducing the circuit current is effective for reducing the radiated electromagnetic noise.
If the circuit can be grounded to the apparatus cabinet, like the secondary circuit, many ground points can be obtained, which reduces the value of the current loop and can thus reduce the amount of the radiated electromagnetic noise.
However, in the patient circuit that cannot be grounded to the apparatus cabinet, the value of the current loop is relatively higher.
Reducing the current to be used in the patient circuit is effective for reducing the amount of the radiated electromagnetic noise in the patient circuit, and the circuit current in conventional apparatus has not been increased.
While means for shielding the patient circuit by the apparatus cabinet, which is a ground (GND) for the secondary circuit may be employed without limiting the circuit current, the size of the apparatus itself increases, which is a problem.
Next, with reference to FIG. 7, a prior art by Japanese Unexamined Patent Application Publication No. 2002-315749 will be described. FIG. 7 is a timing chart showing a process of creating a transmit waveform.
An ultrasonic endoscope has a single ultrasonic transducer at the distal end of the endoscope.
The ultrasonic transducer is rotated about the axis of the endoscope insertion section by rotating rotational driving power, and radial scanning of ultrasonic wave is performed with the rotational scanning.
When the radial scanning is performed, a synchronizing signal (A-phase trigger) is transmitted from the ultrasonic endoscope to the ultrasonic observation apparatus in synchronization with the rotation of the ultrasonic transducer.
The A-phase trigger generates 512 pulses, for example, during one rotation of the ultrasonic transducer in the radial direction.
The ultrasonic observation apparatus supplies transmit signals (transmit pulses) in synchronization with the 512 pulses to the ultrasonic transducer, obtains received echoes thereof and creates one image.
In the prior art, transmit pulses of two burst waves are outputted by handling the A-phase trigger as the synchronizing signal.
The process for outputting two burst waves is shown in FIG. 7.
By handling the A-phase trigger in FIG. 7 as the synchronizing signal, a pulse of a uniform pulse width is created.
The generated pulses are sequentially delayed as the timing signals shown in FIG. 7 by eight delay elements (which will be abbreviated simply to “delay” below) D1 to D8.
The delays D1 and D2 create the first wave of the synthesized pulse P1.
The delays D5 and D6 create the second wave of the synthesized pulse P1.
The delays D3 and D4 create the first wave of the synthesized pulse P2.
The delays D7 and D8 create the second wave of the synthesized pulse P2.
The synthesized pulses P1 and P2 generated by the delays D1 to D8 are inversed, added and synthesized to obtain the output of the transmit pulse of two burst waves.
In order to obtain the output of the burst waves from the synthesized pulses P1 and P2, waveforms switched by a field-effect transistor (FET) are synthesized by using a transformer. The amplitude of the output of the burst waves is about 200 Vp-p.
However, in the prior art case in FIG. 7, obtaining two burst waves, for example, as described above, requires eight delay lines.
Furthermore, more delay lines are required for increasing the number of the burst waves for the purpose of improving the sensitivity by a transmit circuit or suppressing the second-order harmonics.
Means for increasing the number of burst waves is disclosed in Japanese Unexamined Patent Application Publication No. 2002-315749 as a technology of using a programmable delay line, feeding back a pulse delayed once to the input side and resetting the set value of the delay line to generate an arbitrary pulse length.
The technology is certainly effective for reducing the number of delay lines.